api: add words to flow segment POSTs related to overlaps

Clients should not POST a flow segment that overlaps with existing
segments. This includes segments with equal timerange where the
second POST can be viewed as a modification.

api/TimeAddressableMediaStore.yaml

@@ -1309,9 +1309,14 @@ paths:
         - All samples in the object SHOULD be used by the segment.
         - The timestamps of each sample in the media object MUST equal its position on the Flow timeline, OR `ts_offset` MUST
           be set such that `media_object_ts + ts_offset = segment_ts`
-        - The timerange of the segment MUST NOT overlap any other segment in the same Flow.
+        - The timerange of the segment MUST NOT overlap any other segment in the same Flow. The behaviour is
+          undefined if there is an overlap with existing segments and a store may return a 400 error response.
         - The `sample_offset` SHOULD be zero.
 
+        If a client needs to modify a Flow segment, e.g. to correct metadata such as the `key_frame_count` or add additional
+        URLs to `get_urls`, then the client SHOULD first delete the existing segment and then write a new one. The behaviour is
+        undefined if the segment exists and a store may return a 400 error response.
+
         Clients are expected to decide how to break content into media objects, however those objects SHOULD be large
         enough to avoid excessive round trip overheads in the underlying store (_e.g._ of the order of several megabytes)
         and where codecs with temporal re-ordering are used, object SHOULD contain complete GOPs or decodable units.